1. Field of Invention
This invention is directed to air conditioning systems and, more particularly, to a make-up air handler and method for supplying outside air to clean environments such as clean rooms in semiconductor manufacture facilities.
2. Description of Related Art
In certain industries such as semiconductor manufacture, it is necessary to maintain highly clean manufacturing environments (clean rooms) to prevent contamination of materials and fabricated devices. Outside air is supplied to clean rooms by make-up air handlers (primary air handlers) which are located external to the clean rooms. Make-up air handlers perform important functions; namely, filtering the outside air to a desired rating; controlling the humidity of the outside air supplied to clean rooms; maintaining a sufficient air pressure in the clean rooms to prevent their contamination by air flow from rooms at lower air pressures; and compensating for the loss of exhausted air from the clean rooms during the operation of processing equipment in the clean rooms. The filtered outside air is passed from make-up air handlers to secondary air handlers which typically include ceiling-mounted filter units in the clean rooms. The secondary air handlers further filter the outside air before it is introduced into the clean rooms, and also circulate and filter the air within the clean rooms to maintain a desired air class.
In the fabrication of semiconductor wafers and microelectronic devices, metallic and metalloid dopant materials are added to the semiconductor materials to achieve desired electrical properties. For example, boron and aluminum are added to silicon to form p-type semiconductor material, and arsenic, antimony and phosphorus are added to silicon to form n-type material. Because very small amounts of dopants can significantly affect the properties of semiconductor materials, it is important that the dopants be added to the semiconductor materials in closely controlled amounts. In addition, it is important that dopants are not unintentionally introduced into the semiconductor materials.
Semiconductor material purity continues to become increasingly important as device size continues to decrease. Parts per million levels of impurities are often intolerably high in device quality material. Accordingly, it is important to eliminate potential sources of dopants and other impurities that may introduce dopants into clean rooms, where such dopants can contaminate semiconductor materials and make the materials unsuitable for their intended uses.
A potential source of dopants and other impurities for clean rooms is the filtered outside air which is supplied into the cleans rooms through air supply systems including make-up air handlers and secondary air handlers. Although these air handlers can filter the air to a high level of cleanliness and achieve low ratings of the air (number of particles of a specified size per unit volume of air) in the clean rooms, the air supply system can itself be a source of dopants introduced into clean rooms.
More particularly, standard high-efficiency particulate air (HEPA) and ultralow penetration air (ULPA) filters used in make-up air handlers comprise boron-containing fibers. Boron can outgas from the filters and enter the air stream during filtration. The entrained boron can be carried downstream into the clean rooms, where it can contaminate semiconductor materials.
There are known filter materials designed for use in secondary air handler systems that are free of certain dopants. For example, U.S. Pat. No. 5,507,847 discloses a filter that is boron-free, although there is no disclosure of any problems associated with the presence of boron in the filter with respect to the contamination of semiconductor materials. The disclosed filter can be used at air velocities less than about 200 ft/min. Although the disclosed filter can provide boron-free filtered air in secondary air handlers, the air supplied downstream to the secondary air handlers from known make-up air handlers can contain boron, which can be introduced into clean rooms despite the presence of the boron-free filters in the secondary air handlers.
Accordingly, in order to eliminate the presence of boron in air supply systems for clean rooms, including the make-up and secondary air handlers, it is desirable to provide a make-up air handler that is not a potential source of boron contamination of the outside air. However, as stated, known filters used in make-up air handlers are not boron-free. In addition, the make-up air handlers typically operate at air velocities of about 500 ft/min. This rating is significantly higher than the rating of the boron-free filter disclosed in U.S. Pat. No. 5,507,847. Consequently, using the disclosed filter in a make-up air handler according to its intended manner of use at such a higher air velocity, substantially exceeding the rated velocity of the filter, would result in substantial increases in the pressure drop across the filter and associated energy losses. Accordingly, it would be energy inefficient to use the disclosed filter in its intended manner in a make-up air handler.
It is also desirable to provide a boron-free ULPA filter in a make-up air handler to prolong the service life of the filters in the secondary air handlers. That is, by removing particles at a high efficiency at the make-up air handler stage, the particulate level in the air carried to the secondary air handlers would be reduced, thus prolonging the service life of the filters in the secondary air handlers. Prolonging the service life of these filters would provide important cost savings associated with replacing the secondary filters when they become clogged. These costs include the substantial cost of the filters, labor costs to replace the filters, and costs due to downtime of the clean rooms during the replacement of filters. In addition, providing a boron-free ULPA filter in a make-up air handler would provide consistent air conditions in the clean rooms, despite changes in the condition of the outside air due to weather and air quality changes.
Thus, there is a need for a make-up air handler for supplying outside air to clean environments such as clean rooms in semiconductor manufacturing facilities that does not introduce undesirable substances such as dopants into the outside air, and that can utilize known filters and not subject such filters to air velocities exceeding their rated velocities, such that the make-up air handler provides the filtering properties of the filters without requiring significantly increased energy consumption by the make-up air handler in order to use the filters.